The prior art has long recognized the usefulness of catalyst systems comprising a metallocene and an aluminum alkyl in the polymerization of olefins at pressures lower than 20 bar. In the past decade catalyst systems comprising metallocenes such as bis-(cyclopentadienyl) titanium or zirconium dialkyls and an aluminum alkyl/water cocatalyst component have been disclosed for the polymerization of ethylene. For example, European Patent Application No. 0035242 discloses a gas phase polymerization process for preparing ethylene and atactic propylene polymers in the presence of a cyclopentadienyl transition metal salt and an alumoxane. See further U.S. Pat. No. 4,431,788 and German Pat. No. 3,240,382, herein incorporated by reference.
While such metallocene alumoxane catalysts exhibit high activity and efficiency for the polymerization of olefins, three factors have prohibited their commercial use as olefinic polymerization catalysts under high pressures and temperatures.
1. Hazard of Alumoxane Production. An alumoxane is formed from the highly rapid and exothermic reaction of an aluminum alkyl with water. Because of the extreme violence of the reaction the alumoxane cocatalyst component has previously been separately prepared by one of two general methods. In one method, referred to as the "wet solvent production method", extremely finely divided water, such as in the form of a humid solvent, is added to a solution of aluminum alkyl in toluene or other aromatic hydrocarbon. The production of an alumoxane by this process requires use of explosion-proof equipment and very close control of the reaction conditions in order to reduce potential fire and explosion hazards The second method is often referred to as the "hydrated salt method". In this process, an aluminum alkyl is reacted with a hydrated salt, such as hydrated copper sulfate. A slurry of finely divided copper sulfate pentahydrate and toluene is formed and mantled under an inert gas. Aluminum alkyl is then slowly added to the slurry with stirring and the reaction mixture is maintained at room temperature for 24 to 48 hours during which a slow hydrolysis occurs by which alumoxane is produced. Although the production of alumoxane by the hydrated salt method significantly reduces the explosion and fire hazard inherent in the wet solvent production method, production of the alumoxane must nevertheless be carried out separately. The process is also slow and produces hazardous wastes that create disposal problems. Further, before the alumoxane can be used for the production of an active catalyst complex the hydrated salt reagent must be separated from the alumoxane to prevent it from becoming entrained in the catalyst complex and thus contaminating any polymer produced therewith.
2. High Cost. Due to the hazard of producing alumoxane, the cost of alumoxane is very high, which makes it less attractive for commercial application.
3. Toxicity of Alumoxane/Aromatic Solution. Methylalumoxane is more soluble in polar hydrocarbon solvent such as aromatic hydrocarbons. Toluene is the most commonly used solvent for alumoxane. The presence of aromatic residues in polymer products is a health concern which mitigates against the commercial application of alumoxane/toluene solutions in the olefin polymerization.
It would be most desirable to devise an economical procedure whereby a metallocene-alumoxane catalyst useful for high temperature and/or high pressure polymerization procedures could be safely and economically produced. Desirably the catalyst should be one prepared without the necessity for using an aromatic solvent and instead can be readily prepared in non-toxic aliphatic hydrocarbon solvents such as isopentane, hexane, heptane, octane and the like.